Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It appears that your calf is suffering from a serious infection in its rear leg which may be strating to go systemic. This means that the infection has entered the blood stream and will infect all of his organs including the intestinal tract which may be causing this blood from his rear. This disease can also cause a condition called DIC or Disseminated intravascular Coagulation which can cause this bleeding. I know this post is late but very aggressive treatment with IV antibiotics and anti inflammatory medications are very important for teh life of thsi calf.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta